1. Field of the Invention
The present invention relates a method for evaluation of metallic impurities in lithographic materials, more particularly to a method for evaluation of metallic impurities in lithographic materials.
2. Description of the Prior Art
As device dimensions are scaled down in ULSI circuits, the purity of process chemicals is becoming a major concern. It is inevitable for the section of incoming quality (IQC) or quality reliability assurance (QRA) to regulate a strict standard method to ensure the reliability of process chemicals. Due to stringent requirements for detection limit and precision in trace metal in the semiconductor industry, the choice of analytical technology poses a challenging problem.
The lithographic materials applied repeatedly onto a wafer surface play a very important role in the semiconductor process. It is interesting to find out trace metallic impurities in lithographic materials. Based on the complex organic matrix in lithographic materials such as BRAC, i-line resist and DUV resist. It is not easy to determine multi-element directly by an instrument. In this work, the lithographic materials are first decomposed by a closed-vessel microwave oven for the determination of metallic impurities of multi-element by ICP-MS.
The conventional method to evaluate metallic impurities in incoming lithographic materials is shown in FIG. 1. A sample is digested with acids on a hot plate to remove the polymer matrix (e.g. C. H. compounds) in this sample. The digested sample is measured by a graphite furnace atomic absorption spectrometer (GFAAS). The problems of this conventional method are described as follows:
(1) The heating efficiency of a hot plate is not good enough for digesting lithographic materials. It could not remove the polymer matrixes of samples thoroughly. The more matrixes left in a sample the more complicated to measure the metallic impurities.
(2) When samples are digested on a hot plate that is in an opened environment the samples are easily contaminated during the digestion thus the analyzed data will be varied.
(3) The throughput of GFAAS is not fast enough which compares with ICP-MS. Only one element could be measured at a time by GFAAS. It takes 1.5 to 2 hours to measure one sample for measuring eight elements by GFAAS.
For the foregoing reasons, it is necessary to develop a new method to improve the efficiency of digestion and measurement to get more precise results.
According to the present invention, a new method is provided for evaluating ratios of metallic impurities in a photoresist that substantially increases the digesting efficiency and throughput. In one embodiment, the photoresist and nitric acid are put into a closed-vessel tube. The mixture in the tube containing both the photoresist and nitric acid is heated by microwave to do the first digestion. Subsequently, hydrogen peroxide is added into the mixed liquid in the former tube and then is heated in order to do the second digestion. The left liquid mixture is pre-concentrated by a hot plate and then to rinse with 1% nitric acid. The solution of 1% nitric acid with the photoresist residues after digestion is measured by ICP-MS to get the concentration of metallic impurities in the sample.